1. Field of the Invention
This invention relates generally to an active front steer vehicle control system and, more particularly, to an active front steer vehicle control system that integrates and weights both yaw rate feedback and side-slip rate feedback.
2. Discussion of the Related Art
Modern vehicles sometimes incorporate active vehicle control systems. One such system is known as a vehicle stability enhancement (VSE) system that assists the vehicle operator in providing vehicle handling on surfaces such as wet or uneven pavement, ice, snow or gravel. The VSE system typically senses wheel skid based on inputs from a wheel speed sensor, a steering angle sensor, a vehicle speed sensor and a vehicle yaw rate sensor. The VSE system uses these inputs to reduce engine torque and apply differential braking to help maintain the vehicle travel along the intended path.
Additionally, active chassis control systems have been emerging in the art that are used in combination with VSE systems. The chassis control systems typically include differential braking control, real-time suspension damping, rear-wheel steering and active front steering control. With the capability of controlling chassis dynamics in real time, the active chassis control systems can be used to enhance the vehicle handling performance.
Another active vehicle control system is known as an active front steering (AFS) system that provides automatic front-wheel steering. AFS systems typically employ a steering actuator system that receives an operator intended steering signal from a hand-wheel sensor, a vehicle speed signal and a vehicle yaw rate signal, and provides a correction to the operator steering signal to cause the vehicle to more closely follow the vehicle operator's intended steering path to increase vehicle stability and handling. The AFS system is able to provide steering corrections much quicker than the vehicle operator's reaction time, so that the amount of operator steering is reduced. The AFS system provides a more direct vehicle steering under normal road conditions at low and medium speeds, reducing operator steering effort. The AFS system also may help to increase vehicle agility in city traffic or during parking maneuvers. The AFS system also provides less direct vehicle steering at higher vehicle speeds.
In a given operating environment, steering stability and performance of a vehicle are largely characterized by the vehicle's understeer and oversteer behavior. The vehicle is in an understeer condition if the vehicle yaw is less than the operator steering input, where turning the steering wheel more does not correct the understeer condition because the wheels are saturated. The vehicle is in an oversteer condition if the vehicle yaw is greater than the operator steering input.
The known methods of yaw-rate feedback control, such as employed by differential braking control, are sometimes not adequate for the active front steer control. When the vehicle fails to attain the desired yaw rate commanded by the vehicle operator to the hand-wheel due to various responses, such as road surface condition, active chassis control will provide a control component to facilitate an increase of the yaw rate for yaw rate feedback control. When applying this control to the active front steering during such a situation, the additional control component that the control system provides will be an additional amount of front wheel steering. However, under certain situations this additional steering input to the front wheels can saturate the front tires under heavy understeer situation, or the vehicle's failure to attain the desired yaw rate as already caused by a heavy understeer induced by the vehicle operator's excessive steering, resulting in an undesired vehicle behavior.
U.S. patent application Ser. No.10,978,982, titled Method and Apparatus for Controlling Active Front Steering, filed Nov. 1, 2004, assigned to the Assignee of this application and herein incorporated by reference, discloses an AFS system that considers the vehicle understeer and oversteer condition. Improvements to the AFS system disclosed in the '982 application can be provided by considering vehicle lateral dynamics or side-slip.